The loss of fire extinguishing and/or pressurizing material in fire extinguishers of the type wherein the contents are stored under pressure and/or exposed to pressure for delivery purposes has long been a significant concern. For once there is undetected loss, and a fire occurs, the ability to control the fire with the fire extinguishing system is considerably undermined in relation to the lost quantity of fire extinguishing material or the inability to pressurize the fire extinguishing material sufficiently to deliver the entire contents of the system. Consequently, the fire protection industry and/or governmental agencies have long required that extinguishers of this type be checked every six months to determine whether a loss has occurred.
Most frequently, the check is accomplished simply by weighing the fire extinguishing vessel and observing any difference between its fully charged weight and the weight at time of weighing. This method of inspection is quite satisfactory where relatively small vessels are employed in the systems. However, in recent years, there has been a trend to increase the size of the vessels containing the extinguishing agent and as the size of the vessels has increased, the impracticality of weighing the same has become apparent.
Consequently, in the case of fire extinguishers employing liquified gaseous agents such as carbon dioxide, Halon 1301, Halon 1211, and the like, various methods other than weighing have been employed in an attempt to determine the liquid level inside the vessel. For example, various approaches have been tried utilizing sonar, radioactive counters, and heat sensitive liquid crystal tapes to find the liquid level which could then be compared to the original fill level so that the losses, if any, could be established. Such procedures have not been widely accepted due to the fact that such extinguishing agents undergo significant liquid volume changes with changes in ambient temperature. Thus, the only accurate comparison that could be made would be if the measurement were taken at a temperature equal to the temperature at which the vessel was originally filled. As such temperature coincidence rarely exists, the various methods based on such an approach have not been entirely successful although they could provide a rough indication of the need for recharging of an extinguishing system when the loss is fairly substantial.
As vessel size increased, the problem of extinguishing agent loss became of even greater importance since extremely large vessels require a great deal of time and effort to remove them from service for the accurate weight inspection approach. As a result, it was proposed to dispose a tube, accessible from the exterior of the vessel, and sealed thereto, into the vessel to be immersed in the liquid therein. From the access opening, a dip stick of non-magnetic material bearing a magnet on one end and calibrated with indicia was then inserted into the tube. Within the vessel, the tube mounted a magnet carried by a float with the result that when the magnet on the dip stick came within close proximity to the magnet on the float, the resultant attraction would prevent the dip stick from falling further into the tube. The incremental indicia on the dip stick could then be read to provide an indication of the liquid level within the vessel. A calibrated curve in the form of a graph was attached to the exterior of the vessel for showing the volume variations that might occur due to changes in ambient temperature.
This approach, while an improvement over prior art practices, is incomplete. The vessels may not always be filled to the same fill density with extinguishing agent and as a consequence, the calibration shown by the graph, while good for one fill density for one type of first extinguishing agent and for one particular level of super-pressurization, if any, would not be accurate if the fill density, the type of extinguishing agent, or the level of super-pressurization were changed during subsequent recharging. Moreover, the very manual nature of the measuring procedure makes it subject to inaccuracies other than those of mere observation due to the human involvement required.